Update 1 October 2010: If you want FULL sync, then Google has updated there sync tool to support Outlook 2010 - please go to the Gmail blog for more info. Comments to this post will also be disabled as there is nothing left to say.
If you are using Google Calendar and want to use it with Outlook 2010, you will find that the Google Calendar Sync tool no longer works 
So what can you do? Well Outlook 2010 supports the iCal format and so does Google Calendar - so you can use that to get a basic sync between the two.
To set this up go to your calendar details page in Google Calendar (Settings link at the top of the window then Calendars and finally click on the calendar name), and at the very bottom of the is two sets of icons one public & one private. Right click the iCal icon from the private and select copy URL.
Now in Outlook, go to your calendar and click the Open Calendar button and select From Internet, now paste the URL in the text box and click OK.
There you go, your Google Calendar is now in Outlook 2010.
Downside to this is you now have two calendars in Outlook to work with, but if you click that little arrow next to the calendar name 
you can have them display as one and you can easy copy/paste between them.